Recently, the IEEE 802.11 wireless LAN has been widely used for wireless Internet access due to simple installation and low cost. Currently, the IEEE 802.11 wireless LAN defines two different media access control methods.
A basic access method uses a distributed coordination function (DCF) which is a general carrier sense multiple access/non collision acknowledgement method. The DFC is a rule which is applied when a wireless medium is accessed through a competition, and may be used for both of an ad-hoc network environment and an infrastructured network environment.
Furthermore, a medium access control method called a point coordination function (PCF) based on polling may be selectively used.
A wireless network has a broadcasting property. When channels in the same band are used, two terminals cannot transmit data at the same time. When two or more terminals transmit data at the same time, collision may occur, and all transmissions may fail.
When the conventional DCF is applied, three kinds of inter-frame spaces (IFS), that is, a short inter-frame space (SIFS), a PCF inter-frame space (PIFS), and a DCF inter-frame space (DIFS) may be used to implement the IEEE 802.11 medium access control method.
The IFS represents a minimum waiting time which is necessarily required until a next frame is transmitted after one frame is transmitted. The relationship among the SIFS, the PIFS, and the DIFS may be set to SIFS<PIFS<DIFS. Since the SIFS is the shortest, the SIFS is used for communication having the highest priority.
Hereafter, a general method for stochastically avoiding a collision which occurs in a wireless environment, using the DCF, will be described as follows.
The DCF uses a binary exponential back-off method, in order to avoid a collision between transmission terminals and to prevent a specific terminal from successively occupying a channel. The binary exponential back-off method provides a delay time until a transmission is retried, when data transmission signals on a transmission medium collide with each other in a wireless LAN, thereby reducing the probability of successive collision.
Before transmitting data, all terminals check whether a transmission medium is occupied by another terminal. At this time, when the transmission medium is not used during the DIFS, a terminal considers that the transmission medium is idle, and immediately transmits data. Another terminal receiving the data immediately transmits an ACK frame after a predetermined time SIFS, and informs the terminal that the transmission was successfully performed. The terminal having succeeded in transmission preferentially selects a backoff number as an arbitrary value smaller than a contention window, regardless of whether the terminal still have data to transmit, and then starts back off.
The SIFS has a smaller value than the DIFS, and prevents transmission of another terminal from interfering with transmission of an ACK frame. When no ACK frame is received, the terminal considers that the transmission failed, and doubles the transmission window. The initial contention window is set to CWmin, and the maximum size of the transmission window is set to CWMax. When transmission succeeds, the transmission window may be reset to CWmin.
When the transmission medium is occupied by another terminal, that is, when the transmission medium is busy, the corresponding terminal selects the backoff number as an arbitrary value smaller than the contention window. At this time, when the transmission medium is not used during the DIFS, the terminal considers that the transmission medium is idle, and starts back off.
When transmission of another terminal is not performed during a predetermined slot time, the terminal decreases the backoff number by one. When the backoff number becomes 0, the terminal transmits data. When the terminal recognizes transmission of another terminal, the back off is stopped.
Then, when the transmission medium is idle during the DIFS, the back off is resumed.
Then, when the backoff number becomes 0 and the transmission medium is idle, the terminal starts transmission.
The conventional DCF has been necessarily employed as a medium access control method of the IEEE 802.11 wireless LAN. In general, however, the conventional DCF exhibits low performance. In particular, as the number of terminals increases, collision frequently occurs. Furthermore, unused back-off slots cause bandwidth waste. Thus, the conventional DCF has a low transmission rate.
In order to solve the above-described problem, various methods have been proposed. However, in a general competition-based distributed medium access control method, a collision and an unused back-off slot are in a trade-off relation. Thus, when one is intended to be reduced, the other is increased. Therefore, such an access method has limitation in improving performance to a desired level.